Is eight hours of sleep enough?

For a long time, the idea of an ideal night’s sleep came down to a single number: eight hours. But a new generation of studies is changing that perspective. Researchers now observe that going to bed and waking up at similar times every day may have an impact on physical and mental health as important as, or even greater than, reaching a given target number of hours of sleep. What’s at stake is not just rest, but keeping the body in sync with its own biological clock.

This shift in perspective is gaining ground at a time when people’s routines are becoming increasingly irregular. Hybrid work, on-call shifts, heavy screen use, meals at varying times and constant connectivity mean that millions of people are constantly shifting their bedtimes and wake times. The result is a body that loses predictability precisely when it needs it most.

The body runs on rhythm, not hours

Neuroscientist Fernando Louzada, full professor and coordinator of the Human Chronobiology Laboratory at the Universidade Federal do Paraná (UFPR), explains that “sleep is perhaps the most evident rhythm in our body, which is rhythmic through and through. Hormones, cells, organs: everything shows a rhythmicity that serves to synchronize how we function. When we keep our sleep regular, we preserve synchronization with the environment’s natural light-dark cycle. And that translates into better health.”

The brain is constantly working to anticipate what lies ahead. Even before dawn, the body begins adjusting hormone production, body temperature and various metabolic processes to prepare for waking up. When schedules change repeatedly, this orchestra loses its beat. Exposure to artificial light at night reduces melatonin production, alters cortisol secretion and triggers a cascade of effects that reaches metabolism, cognition, immunity, emotional regulation and even gastrointestinal function.

More important than the clock hitting eight hours

For physician Monica Andersen, director of Teaching and Research at the Instituto do Sono and a member of the Academia Brasileira de Ciências (Brazilian Academy of Sciences), regularity now ranks first among the classic sleep hygiene measures. She lays it out: “If I had to set an order of priorities, I would put regular schedules first, followed by the number of hours slept and making sure sleep quality isn’t compromised. When we have these three characteristics, we promote much better physical and mental health.”

This doesn’t mean sleep duration has stopped mattering. What has changed is the understanding that there is no universal ideal amount. The recommendation of seven to nine hours works for a portion of the population, but some people naturally need less time and others need more hours to function well throughout the day. “People often make the mistake of turning averages into norms,” Louzada sums up. According to him, the best indicator is still daily functioning: energy, attention, stable mood, weight and metabolism without significant changes, cognitive performance and the absence of excessive sleepiness.

The price of “social jet lag”

One of the most common consequences of irregular sleep is so-called social jet lag, a situation in which a person wakes up early during the week and tries to make up for the fatigue by sleeping in much later on weekends. Although recovering part of the lost sleep may bring some immediate benefit, the habit widens the gap between sleep and wake times, further disrupting the biological clock.

“Sleep is never made up in the same proportion. We can neither make up for it nor maintain rhythmicity. The consequences can be very bad for the body, with metabolic changes, weight gain, intestinal, sexual and skin problems, poor concentration and memory, mood swings and greater risks to mental health,” says Monica Andersen.

The most recent scientific findings reinforce this view. A study published in 2025, with nearly 80,000 UK Biobank participants followed for about seven years, showed that people with more regular sleep had a 38% lower risk of developing depression and a 33% lower risk of anxiety, even when their sleep duration was similar to that of individuals with irregular schedules. Another analysis, which pooled data from eight international studies, concluded that variability in bedtimes and wake times is consistently associated with symptoms of anxiety, depression and insomnia, regardless of average sleep time.

The challenge of modern life

Science is also beginning to discuss a delicate balance: respecting individual differences without giving up regularity. People with later chronotypes, for example, naturally tend to go to bed and wake up later. For Louzada, the solution isn’t to impose rigid schedules on everyone, but to create routines compatible with those differences whenever possible: “The challenge for today’s society is to make work and school schedules more flexible without losing regularity. The body needs time anchors: relatively stable times for sleeping, waking up, eating meals, exercising and maintaining social interaction.”

For people who work shifts or on-call schedules, the situation is more complex. Daytime sleep rarely reaches the same quality as nighttime sleep, even when the duration is similar. Even so, experts recommend strategies that can reduce some of the harm, such as keeping consistent schedules whenever possible, getting natural light exposure at the right times, maintaining a balanced diet and investigating sleep disorders when symptoms such as fatigue, difficulty concentrating and mood changes persist.

In the world we live in today, the question “How many hours do you sleep a night?” is no longer enough. Science suggests a broader question: “Do you go to sleep at roughly the same time every day?” In a society marked by unpredictability, cultivating a consistent routine may be one of the simplest and most powerful interventions to protect the brain, metabolism and mental health throughout life.

Scientific studies cited or used for supplementary information:

- “Regular sleep patterns, not just duration, critical for mental health: association of accelerometer-derived sleep regularity with incident depression and anxiety” – Available at https://pubmed.ncbi.nlm.nih.gov/40814280/ 

- “Irregular sleep is linked to poorer mental health: A pooled analysis of eight studies” – Available at https://pubmed.ncbi.nlm.nih.gov/38704353/

- “Routine regularity during a global pandemic: Impact on mental health outcomes and influence of chronotype” – Available at https://pubmed.ncbi.nlm.nih.gov/38380627/